Src-transformation of mouse fibroblasts induces a Ca(2+)-activated K+, current without changing the T-type Ca2+ current.

Membrane currents of src-transformed NIH3T3 mouse fibroblasts were analyzed in comparison with their non-transformed counterparts using the patch-clamp technique. Normal NIH3T3 cells exhibit two types of Ca2+ currents and a membrane current of ohmic behaviour (current amplitude 135 pA at +30 mV) that can partially be blocked by Cd2+. Src-transformed NIH3T3 cells show an additional membrane current that becomes activated after the establishment of the whole-cell configuration with a maximum amplitude of 1040 pA at +30 mV within 30-60 s. This current then inactivates irreversibly within 5-10 min. The additional current is highly K(+)-selective and Ca(2+)-dependent but voltage-independent. It can be blocked by charybdotoxin (IC50 = 20 nM) and by internal tetraethylammonium (TEA; IC50 = 2.9 mM), but it is not sensitive to external TEA (up to 30 mM). Single-channel analysis revealed only one K+ channel type with a conductance of 37 pS at negative potentials and 18 pS at positive potentials (in symmetrical 145 mM K+ solutions), a voltage-independent open-state probability of 0.6 and the same pharmacological properties as the macroscopic KCa current. The properties of the KCa current and the underlying channels of src-transformed NIH3T3 cells are identical to those observed in ras-transformed NIH3T3 cells. In contrast, src- or ras-transformation affects differently the voltage-dependent, transient (T-type) Ca2+ current. While ras-transformation of NIH3T3 cells suppresses their T-type Ca2+ current, this current remains unchanged in src-transformed NIH3T3 cells.